Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
However, in modern low voltage electron microscopes, aberrations limit the achievable resolution to approximately 3 nm for 1 keV electron energy. Especially for low energy application, it is therefore desirable to reduce chromatic aberrations. The diameter of the aberration disc of the chromatic aberration in the Gaussian image plane of an objective is proportional to the relative energy width ΔE/E of the charged particle beam.
The electrons in an electron beam column are not monochromatic because of the emission process and the Boersch effect, that is, the broadening of the energy distribution because of stochastic Coulomb interaction so that the relative energy width is increased. In view of the above, the energy width ΔE amounts to approximately 0.5 to 1 eV or even higher, in dependence upon the beam current.
A further minimization of the chromatic aberration based on the focusing properties of, for example, the objective lens is difficult. For this reason, it is already known to utilize monochromators, in order to further increase the resolution. Thereby, the energy width ΔE of the electron beam, which is processed subsequently by the downstream electron-optical imaging system, can be reduced.
Wien filters are known as monochromators for charged particles wherein an electrostatic dipole field and a magnetic dipole field are superposed perpendicularly to each other.
As an example, patent publication U.S. Pat. No. 6,489,621 (Frosien et al.) shows a device for reducing the energy width of a particle beam with a 1st and a 2nd Wien filter for dispersing the particle beam depending on the energy of the particles, and an aperture for selecting the particles within a certain reduced energy width. Thereby, a disadvantageous crossover can be avoided for small dispersion values.
However, there is a necessity for a system suitable for an increased dispersion of a charged particle beam energy width reduction system.